Generation and precise control of low and medium energy pulsed electron beams is required for many industrial, medical, and research applications, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), and horizontal/vertical accelerator-based beamlines (HAB/VAB), as well as relevant experimental analytical methods that use electron beams in SEM or TEM, or in HAB/VAB as probes.
In research, pulsed electron beams with ultrashort pulse durations are used for investigating dynamic processes in a variety of materials. Frequently, the electron beams are combined with other primary excitation probes such as laser beams or other photon-based probes such as X-ray beams. An example would be the “pump-probe” class of experiments.
One approach for generating electron beam pulses of a specific length and charge (i.e. intensity) in a periodic sequence is to create electron pulses directly on the surface of an electron source (cathode) by exciting the electrons using either a laser or heat combined with an external electric field.
If a laser is used as the excitation method, the sequences of electron pulses are controlled by adjusting the wavelength, power, and/or temporal structure (pulse length and repetition frequency) of the laser photon pulses. For example, if a combination of femtosecond lasers and photocathode electron emitters is used, the electron pulse lengths are strictly determined by the pulse lengths of the fs-laser and the response time of the photocathode. Using this approach, it is possible to routinely obtain pulse lengths as short as 100 femtoseconds (“fs”) or less.
However, high repetition rates, defined herein as being repetition rates of at least 1 GHz or higher, are simply not available for laser-excited electron beams, because modern lasers are only capable of repetition rates on the order of 100 MHz or less. For example, the commercially available UTEM (Ultrafast TEM) is a stroboscopic pump-probe method, where the pump and the probe signals are both laser-actuated. Most laser-driven processes, including many processes that are driven electrically, magnetically, or both, can be cycled indefinitely at frequencies above 1 GHz, thus enabling truly in operando microscopy, with the most notable example being switching in a semiconductor device. However, in UTEM, data are repeatedly collected over extended periods of time, and thermal load from the pump laser must be managed so that a process under study is not irreversibly damaged. Therefore, even though lasers with higher repetition rates are available, UTEM systems typically operate at much less than 0.1 GHz, and sometimes even at about 0.1 MHz, depending on the experiment.
In addition, it is often important in experimental systems to provide flexible and simple solutions for switching between continuous and pulsed beam modes. If the combination of a photocathode and an fs-laser is used for pulsed beam generation, then the required continuous beam must be generated using a separate thermionic or field emission source.
On the other hand, if heat combined with an external electric field is used as the excitation method, then the sequences of electron pulses are controlled by the electric field strengths and the temporal structure (pulse length and repetition frequency) of the electric field pulses.
Still another approach for generating pulsed electron beams is to mechanically or electromagnetically block and unblock (i.e. “chop”) a continuous electron beam at a desired periodicity, according to the desired electron pulse timing. Typically, a transverse oscillation is imposed onto the beam, and then an aperture is used to chop the oscillating beam into pulses. Approaches that use deflecting cavity technology for chopping electron beams of tens of kV in the GHz frequency range have been known since the 1970's.
According to this approach, a continuous beam of electrons is directed through a device, referred to herein as an electro-magnetic mechanical pulser (EMMP), that operates to chop the beam and collimate the output. The EMMP includes a “kicker” that uses radio frequency energy to impose transverse oscillations onto the beam according to at least one of a time-varying electric field and a time-varying magnetic field generated within the kicker, after which an aperture “chops” the laterally oscillated beam into pulses. The RF is generated in the kicker in a “transverse” mode, meaning that its electric and magnetic field components oscillate transverse to the beam propagation direction. More specifically, the electric and magnetic components of the RF wave propagate in orthogonal planes that contain the long axis of the EMMP along which the electrons propagate.
One possibility for implementing an EMMP kicker is to use a “stripline.” As is generally known, in a metallic traveling wave stripline, which in its simplest form comprises two flat metallic parallel slabs, if the medium between two slabs is vacuum or air, then the phase velocity of the RF electromagnetic wave will travel along the stripline at the speed of light. However, for many applications the electrons travel much more slowly. For example, in TEM applications the energy of the electrons is typically in the range of 100 keV to 300 keV, whereby the electron beam speed is around 2.1×108 m/s, which is only about 70% of the speed of light.
In many EMMP applications, it is therefore necessary to limit the interaction time between the RF wave and the electrons, because otherwise the electrons will experience a phase slippage, which will mean that the overall applied kicking force will be greatly reduced or even cancelled. For this reason, among others, current approaches often employ just one single-cell deflecting cavity, and are typically limited to pulse lengths of 1 picosecond (“ps”) at best and repetition rates of 1 GHz or less, which cannot be changed or tuned. Furthermore, these approaches are only applicable for generating low energy electron beams having energies of less than 100 kilo-electron Volts (“keV”). Perhaps even more importantly, these approaches typically result in very extensive electron beam quality deterioration in both the transverse direction (beam diameter and divergence) and longitudinal direction (temporal coherence).
An EMMP that implements a transverse deflecting cavity (TDC-EMMP) is disclosed in U.S. Pat. No. 9,697,982, and in an article published in Ultramicroscopy 161 (2016) 130-136, both of which are incorporated by reference herein in their entirety for all purposes. The TDC-EMMP disclosed in these references avoids the problem of phase velocity mismatch by creating a standing electromagnetic wave within the transverse deflecting cavity, rather than a traveling wave, as the mechanism to impose a spatial oscillation onto a continuous input electron beam. This TDC-EMMP approach is able to generate electron beams that can be pulsed at a high duty cycle with pulsing rates greater than 1 GHz and with minimal transverse and longitudinal dispersion. The spatially oscillating beam is then applied to an adjustable Chopping Collimating Aperture (“CCA”) so as to break the beam into a series of pulses, after which a dispersion suppressing section comprising a plurality of cavity resonators and/or magnetic quadrupoles is used to suppress temporal and spatial dispersion of the pulsed beam.
FIG. 1A is a conceptual diagram that illustrates the fundamental concepts underlying the TDC-EMMP approach. In the illustrated example, an initially continuous, “DC” electron beam 100 is transversely modulated into a sinusoid 110 as it passes through a vacuum-filled TDC 102 which is operated at a resonant frequency that lies within a range between 1 GHz and 10 GHz. The amplitude of the sinusoid 110 grows as the modulated beam propagates, and then the beam 110 impinges upon a chopping, collimating aperture, or “CCA” 104, having an opening 106 that is adjustable between 10 μm and 200 μm. The CCA “chops” the beam into pulses 108 that emerge from the CCA at an ultrahigh repetition rate that is twice the TDC modulation rate, because the pulses 108 are produced by cutting the sinusoid 110 of the beam modulation on both the up-swing and the down-swing. The aperture opening 106 and the modulating field of the TDC work together to tune the pulse lengths to between 100 fs and 10 ps, resulting in duty cycles of the TDC-EMMP device of less than or equal to 20%.
After the beam 100 has been chopped into pulses 108, if nothing further were done, both the longitudinal and lateral divergence of the stream of pulses 108 would increase. In other words, the pulses would get longer (temporal divergence in the propagation or “z” direction) and would spread out (spatial dispersion in the x and y directions). So as to avoid this, as shown in FIG. 1A, additional components 112, 114 are included in a divergence suppressing section downstream of the CCA 110 that reverses and suppresses this divergence. In the example of FIG. 1A, the divergence suppressing section includes an additional, demodulating, TDC 114, which is identical in design to the modulating TDC 102, as well as a magnetic quadrupole 112. Additional details as to the features and underlying principles of the TDC-EMMP are presented in application Ser. No. 15/091,639, and in Ultramicroscopy 161 (2016) 130-136.
With reference to FIG. 1B, in a similar implementation the TDC-EMMP 120 includes a first electromagnetic kicker 102, a collimating aperture 112, a first magnetic quadrupole 112, a second “mirror” electromagnetic kicker 114 that functions as a demodulating element, a second magnetic quadrupole 116, and a third magnetic quadrupole 118. The incoming longitudinal DC electron beam 100 is directed along the optical axis (dashed line in FIG. 1B). At the entrance of the TDC-EMMP 120, a transverse sinusoidal momentum is imparted to the DC beam 100 by the electromagnetic field generated by the first beam kicker 102. Since the electromagnetic field oscillates in the transverse direction, its perpendicular electric and magnetic components vary with time, such that the modulation force that is applied to each incoming electron 100 depends on the time at which it arrives in the TDC kicker 102.
The amplitude of the sinusoid grows in the transverse (horizontal in FIG. 1B) direction as the modulated beam propagates along the optical axis 100 (downward in the figure). When the beam reaches the collimating aperture 104, which is placed on the optical axis 100 downstream of the first kicker 102, the slit in the aperture 104 chops the beam 100 and converts it into a pulsed sequence. However, after passing through the aperture the beam will expand, and both the beam size and divergence will increase. As shown in FIG. 1B, the addition of quadrupole magnets 112, 116, 118 and a second “mirror” beam kicker 114 can demodulate the beam 100 and reduce both its emittance growth and energy spread (i.e. both the spatial and temporal coherence of the beam).
According to the TDC-EMMP approach, the pulse length, and therefore the duty cycle of the pulses, can be adjusted by varying the RF amplitude. However, the wavelength of the standing wave within the TDC kicker is fixed by the dimensions of the TDC. Accordingly, the pulsing rate is adjustable only by varying the electron velocity, and cannot be adjusted independently.
Another approach to EMMP electron beam pulsing is to implement a traveling RF wave in a stripline that is configured to reduce the phase velocity of the RF as it propagates through the kicker. U.S. patent application Ser. No. 15/368,051, included herein by reference in its entirety for all purposes, discloses such an approach, whereby a traveling RF wave is generated in the kicker, but the RF wave is propagated through a dielectric, causing the phase velocity of the RF wave to be slower than the speed of light, and thereby allowing the electron velocity to be matched to the RF phase velocity. More specifically, according to this approach the “kicker” is a Traveling Wave Transmission Stripline (TWTS) that is terminated by an impedance load. In exemplary implementations of this approach, the TWTS kicker is a hollow continuous tube that is dielectric-filled. The electron beam propagates through the hollow center of the tube, while transverse-mode RF waves simultaneously propagate through the tube. As noted above, the dielectric serves to reduce the phase velocity of RF to a sub-light velocity that can be matched to a velocity of the electrons in the beam.
The RF phase velocity in the TWTS kicker is independent of the RF frequency, such that the modulation rate of the beam and the resulting pulse rate can be tuned over a very wide range by adjusting the RF frequency to a desired value. Independently, the amplitude of the electron beam modulation, and thereby the pulse width, and consequently the pulse duty cycle, can be varied by varying the amplitude of the applied RF. In embodiments, the divergence suppressing section according to this approach includes a “mirror” dielectric TWTS that functions to suppress residual transverse oscillation and divergence of the pulsed beam.
The TWTS-EMMP offers advantages of independent, continuous adjustment of the pulse width and duty cycle over an ultra-broad operating bandwidth, and also includes the advantage of fabrication simplicity. In embodiments, this allows the radiation dose rate to be reduced below a damage threshold level of the measurement sample, while maintaining a high pulse repetition rate so as to rapidly accumulate data. Important applications include electron tomography of cellular structures over a wide range of spatial and temporal scales. The TWTS-EMMP can also be advantageous for enabling a high frequency stroboscopic mode in Transmission Electron Microscope (TEM) applications, whereby the dose rate of the TEM can be varied. Low dose rate TEM can be crucial, for example, when examining biological samples that are vulnerable to radiation damage caused by energetic electrons.
It should be noted that the term “duty cycle” is defined herein as being the ratio of the electron beam pulse width divided by the time between successive electron beam pulses. It should further be noted that the term “continuous variation” and derivatives thereof are used herein to refer to parameters that can be adjusted smoothly throughout their defined ranges, without gaps.
While the TWTS-EMMP provides many advantages over previous approaches, under some circumstances the dielectric that is included in the TWTS-EMMP can be subject to “electron charging,” whereby the dielectric acquires an electric charge due to impacts by incoming electrons. While this can be alleviated by applying a thin film conductive coating to the dielectric, the application of such a conducting film can result in loss of electromagnetic energy and reduced system efficiency.
What is needed, therefore, is an alternative to the TWTS-EMMP that provides many of the advantages of the TWTS-EMMP, including production of a pulsed electron beam that is independently and continuously tunable over a wide range of pulse repetition rates and pulse duty cycles, but is not subject to electron charging.